Network technology has become very common due to the recent massive price decline and significantly improved communication speed. Most offices are equipped with a plurality of computers, and it is common thing to network these computers. Also, with wireless LAN technology, services that enable Internet access from public places such as libraries and cafes, as well as offices and homes, has now become available. At present, there are a large number of Internet users and a large number of emails sent and received from hotspots where Internet access points through wireless LAN are available.
Although wireless LAN technology is widely used for laptop computers and PDAs, it has hardly been incorporated into mobile phones up to now. This is mostly because wireless LAN consumes a great amount of power. In order to reduce power consumption, IEEE802.11 specifications, the standard for wireless LAN, designates a method called Power Saving Protocol (PSP). According to this method, a communication station that does not serve as an access point monitors the network only once per interval between N beacons. The communication station goes into sleep mode while not monitoring the network, so that power consumption can be reduced. Any data reaching the access point during sleep mode is temporarily stored by the access point and then passed to the relevant communication station when communication begins again. In addition, IEEE802.11e specifications that are currently under consideration specify a new method called Automatic Power Save Delivery (APSD). APSD follows same basic concept as PSP, reducing power consumption by minimizing the network monitoring time of the wireless LAN device and switching the device to sleep mode when not monitoring the network.
On the other hand, in the technical field of low-power LSI design, which has little to do with wireless LAN technology, a totally different approach has been employed to reduce the power consumption for information processing circuits. This technology is called “voltage hopping”, wherein the driving frequency and supply voltage of the information processing IC is varied depending on the load status. The IC power consumption is proportional to the driving frequency and also the supply voltage squared; therefore, dynamic control of the two can effectively reduce the power consumption.
For future generations of wireless LAN, it is expected that real-time applications such as VoIP (Voice over IP) technology or VOD (Video On Demand) technology will be widely used. In such real-time applications, it will be important to secure QoS (quality of service) in communication. Accordingly, for future generations of wireless LAN, it is necessary to overcome the challenging issue of reducing system power consumption while securing communication quality.
Reference 1: IEEE802.11e draft
Reference 2: by Hiroshi Kawaguchi, et al., “CVS (Cooperative Voltage Scaling) and VDD hopping among OS, application and hardware for low-power, real-time embedded systems,” Technical Report of IEICE, May 2001